Engine valve driving systems are known which have a plurality of cams for driving an intake and/or exhaust valve to change valve timing and/or the quality of the valve lift in accordance with an engine operating condition. One such engine valve driving system is described in unexamined Japanese Utility Model Publication No. 58605/1986. This system provides a first rocker arm pivotally mounted on a rocker shaft for driving an intake valve under low speed engine operating conditions, in accordance with the movements of a low speed cam having a cam lobe suitable for low speed engine operation. Beside the first rocker arm is a second rocker arm pivotally mounted on a rocker shaft for driving the intake valve under high speed engine operating conditions, in accordance with the movements of a high speed cam having a cam lobe suitable for high speed cam operation. Engaging means engages the second rocker arm with the first rocker arm under high engine RPM and disengages the second rocker arm from the first rocker arm under low engine RPM.
An operating oil supply system for another type of engine valve driving apparatus is described in unexamined Japanese Patent Publication No. 59-231118 (U.S. Pat. No. 4,537,165). This reference discloses an operating oil supply system for a valve actuating mechanism having a stopping function, with a plunger selectively positioned by operating oil pressure controlled by an oil pressure control valve, which is actuated in response to engine operating conditions.
However, utilizing the above-mentioned systems requires rapid shifting of the plunger between an inoperative and operative position, and it is usually necessary to shift into the operative position several times before the oil supply passage is filled and the plunger beings to work, which can be relatively time-consuming.
In order to overcome this problem, an accumulator or similar mechanism is used with the oil pressure control apparatus. However, in an internal combustion engine, it is still difficult to raise the oil pressure to a sufficient level, especially when the engine is running at low speed. Nor does lowering the operating pressure of the plunger serve as an appropriate correction, since such action would require weakening the force of a return spring for the plunger, thereby requiring even more time to shift the plunger from the operative to inoperative position. Further, where the oil passage serves both as an operating oil and lubricating oil supply passage, it is difficult to lower the set pressure of the plunger below that of a predetermined limit value for preventing insufficient oil supply.